1. Field of the Invention
The present invention relates to laser based systems and methods. More specifically, the present invention relates to systems and methods for synthetic aperture ladar.
2. Description of the Related Art
Long range imaging or mapping has become of great interest for both defense and commercial applications. In the defense world, high resolution target imaging allows for target identification at safe ranges beyond weapon capabilities. In defense and also commercial, there is a desire to perform terrain mapping for high resolution topography.
Prior approaches to long range imaging have used radar systems, particularly synthetic aperture radar (SAR). However, the angular resolution achievable with radar is limited by the fact that radar has relatively long wavelengths (compared to optical frequencies).
Ladars have been identified as having unique capabilities for high resolution 3-D imaging. A ladar system often includes a sensor suite mounted on a satellite, missile system, or aircraft. The sensor suite has one or more fixed physical apertures through which a ladar system views a scene. A ladar system views a scene by transmitting a laser through the aperture toward the scene. The laser reflects off the scene, producing a laser return that is detected by the ladar system. Many conventional radar and ladar systems measure the intensity of the return beam and the round trip delay from transmission to detection, which yields the distance (range) to the scene. Laser return intensity and range information may be combined with other image information to facilitate target tracking, terrain mapping, and so on.
In a conventional ladar imaging system, image cross-resolution is limited by the size of the ladar system aperture. Very large and expensive apertures are required to obtain sufficient resolution for many current long-range imaging and mapping applications. This is particularly problematic for ladar systems employed in satellites or missile systems, which have prohibitive space constraints and require long-range viewing capabilities.
To reduce aperture-size requirements, synthetic aperture radar and ladar systems are employed. In a synthetic aperture ladar (SAL) system, additional information about the scene is obtained by changing the viewing angle of the scene. This additional information, called cross-range information, is contained in Doppler frequency shifts detected in the laser return caused by the transmit laser striking various features of the scene at different angles. Cross-range information indicates the relative angular position of certain scene features associated with a given range or distance from the ladar system. The cross-range information is combined with range information to yield an accurate scene profile to enhance the image of the scene.
High resolution applications operating at a range of approximately 100 kilometers, an eye-safe laser wavelength of 1.5×10−6 m, and a typical cross resolution of 20 cm, require a conventional aperture of approximately 75 cm, which is prohibitively large and expensive for many applications. The large apertures are also undesirably sensitive to thermal and gravitational distortions. An analogous synthetic aperture ladar system on a platform traveling at, for example, 100 m/s would require a measuring time of 7.5 milliseconds (ms) to cover the required 75 cm aperture.
Conventional synthetic aperture ladar systems require that the laser transmitter produce a high-power waveform that is coherent for the entire duration of the measuring time during which the laser return is detected. The high power is often required to reach long ranges of interest. Typically, coherent waveforms longer than a fraction of a millisecond are difficult to achieve, especially at high power levels. In addition to coherence time and high power, the transmitted waveform requires high bandwidth to achieve high down-range resolution, yielding typical bandwidth-time products (BT) greater than 300,000. This implies that the transmitted waveform must be accurate (phase coherent) to 1/300,000 (1/BT). Consequently, conventional synthetic aperture ladar systems have generally been unsuccessful in achieving this bandwidth time product.
Previous synthetic aperture ladar systems could not maintain transmitter coherence for sufficient duration to accurately measure a scene. Accurate synthetic aperture measurements require relatively high beam pulse energy for which coherence is difficult to maintain. Prior attempts at synthetic aperture ladar have tried using the same waveform used in synthetic aperture radar systems, a train of FM chirped waveforms. Each chirp waveform has to be coherent with the next (i.e.—have the same optical phase), but this was impossible with intracavity modulation, and out-of-cavity modulation requires huge sizes and voltages, making the ladars impractical for flight units.
Hence, there is a need in the art for a laser transmitter capable of producing the coherence, high power, and high bandwidth required for use in synthetic aperture ladar applications.